1. Field of the Invention
This invention relates to digital-to-analog converters of the segment type formed on a single monolithic IC chip. More particularly, this invention relates to such converters having two cascaded stages, the first stage resolving a set of higher-order bits into a corresponding analog signal, and the second stage resolving the remaining, lower-order bits to produce a second analog signal to be combined with the first stage analog signal.
2. Description of the Prior Art
Susset U.S. Pat. No. 3,997,892 shows a two-stage cascaded converter wherein the first stage includes a resistor-string DAC (D/A converter) to produce a first voltage corresponding to a set of higher-order input bits. The second converter stage is another resistor-string DAC arranged to produce a second voltage corresponding to a set of lower-order bits. The voltage across a selected resistor of the first stage is applied to the ends of the resistor string of the second stage, so that the latter stage produces an output effectively interpolating the selected first-stage segment voltage in accordance with the lower-order bits.
Converters such as shown by Susset have an important advantage in that they are inherently capable of monotonic performance. However, the Susset converter is practical only for use in relatively low-resolution applications. This is because the selector switch system used to make connection to the resistor string would become prohibitively large and complex for a high-resolution monolithic converter such as one capable of resolving a 16-bit input word. For example, the first stage of such a converter typically would have a 256-R resistor string. The complexity of the switch selector system for such a resistor string especially results from the fact that it must be able to select any pair of adjacent voltage taps of the string to produce the segment voltage for the second stage converter.
The prior art techniques used in cascaded segment converters suffer from still other disadvantages when applied to high-resolution, high performance devices developed on a single IC chip. For example, prior resistor-string arrangements have not been found suitable for achieving satisfactory linearity and other desired performance characteristics in a 16-bit converter.